Check valves permit flow in one direction only, and close automatically if flow reverses.
This non return valve has an internal brass swinging flap and is designed to allow inspection and cleaning without the need to remove it from pipework.
1/2 inch BSP female threaded connections on each end.
BSP (British Standard Pipe Thread) sizes don’t match up with what you would measure using a ruler. Have a look at the BSPT link at the bottom of this page for more information about how plumbing threads are measured.